Interstage question in 6ac5G design

[FW]

Hello,

I need some help about the following:

Recently I acquired a pair of HP 9100-1390 interstage looking transformers in an electronic dump shop. The DC resistance of both sections measured 365 ohm so I was hoping to have a 1:1 interstage. A google search did not show any results apart from that the trannies were most likely manufactured by Paeco.

In a brave attempt to do something with the transformers I tried to improve a dynamic coupled prototype SE design with 6J5-6G6G (trioded)-6AC5G to a design with the transformer between the 6G6G and de 6AC5G.



After powering up the amp the voltage swing at the 6AC5G grid was very low (0.2 Vpp) while at the primary 6G6G site a swing of more than 80V was measured. Exchanging primary and secondary show the same results. Apart from the attenuation between primary and secondary I was surprised by the fact that exchanging primary and secondary did not results in a change.

My question: Is it the “interstage” transformer that seems not applicable or could it be something in the design that I have overlooked?

Thanks in advance

Frans

[tubelab.com]

I am not familiar with the transformers that you mention, but it is entirely possible that they were not made to handle any DC current through the primary. The DC flowing through the primary may have fully saturated the core prohibiting any transformer action.

Test the transformer with a signal source (audio oscillator, computer sound card output, or CD player with a test disc) to see if it works over the audio range with no DC flowing through it. If it does you will need some sort of parafeed arangement, or an equal and opposite current through the secondary. Either way it complicated your amp design.

[astouffer]

It looks like the output tube is running positive bias, is that correct?

[Osprey]

I would try it with a signal generator also to see what you are dealing with. Try different loads on the primary and secondary.

I also think it is presuming a lot to think it will carry any DC. I would try to parallel feed the primary and ground the secondary and use cathode bias for the output tube until you know the operating parameters of the transformer.

[FW]

Yes, that's correct. The 6AC5 is a small pos. grid triode.